Method and Device for Treating Anode Slime

ABSTRACT

The method and furnace according to the invention enable a continuous processing of anode slime and are particularly suited to be connected to a process where anode slime is treated by hydrometallurgic methods after roasting. In the method according to the invention, the slime containing valuable metals and selenium is dried, roasted, sulfatized and cooled. The method includes steps to be carried out in succession, in continuous operation, so that the slime forms a slime layer on the conveyor and is conveyed to be treated in successive drying, roasting, sulfatizing and sulfuric acid removal and cooling units.

The invention relates to the metallurgic treatment of solid raw materialcontaining valuable metals and selenium. More precisely, the inventionrelates to a method and device for treating anode slime obtained fromcopper electrolysis.

In copper electrolysis, the insoluble element of the anodes falls on thebottom of the electrolytic tank as anode slime, from where it isrecovered when the anodes are replaced. In addition to copper andnickel, anode slime also contains metals that are more precious thancopper, for example tellurium, selenium, gold, silver and platinummetals, as well as impurities such as arsenic, sulfur, antimony, bismuthand lead. Valuable metals and impurities are separated in the treatmentof anode slime.

Known anode slime treatment methods are combinations of hydrometallurgicand pyrometallurgic methods. Generally anode slime contains thefollowing ingredients: Cu, Ag, Au, Cl, Sb, Pb, Ni, Ba, Pt, Pd, As, Bi.The initial humidity of the slime is typically 10-30%.

In a known anode slime treatment process, from the slime there are firstremoved copper and nickel, then silver, next gold, and finally platinummetals are removed separately. Selenium is generally separated byroasting after removing copper and nickel. The separation of copper andnickel is carried out by leaching at a high pressure and temperature inthe presence of sulfuric acid and oxygen, so that copper, nickel andpart of the arsenic and tellurium are dissolved. After copper removal,selenium is removed by roasting. The currently used roasting process isa batch process, where slime is roasted at the temperature of about450-600° C. In that case selenium is removed as SeO₂ gas.

The drawback with known roasting processes and furnaces is the limitedmaterial transfer into and from the slime. Owing to their thickness,slime layers formed on top of the trays of furnaces operated accordingto the batch principle restrict the degree of usage of the roastinggases. The proceeding of the roasting gases through the thick layer isslow, and respectively the exhaustion of selenium dioxide from the slimecake takes place slowly and restricts the speed of the process. Heattransfer problems are also connected to the prior art. It is difficultto control the temperature in the various steps of the process and indifferent parts of the furnace. The slime temperature easily rises toohigh, in which case the slime cake is sintered, and the materialtransfer into and from the slime is slowed down and may be evencompletely interrupted.

From the patent publication U.S. Pat. No. 4,228,133, there is known acontinuously operated anode slime roasting furnace, where the slime isfed into a furnace comprising one reaction space, where the slurry istransferred from tray to tray on top of moving trays. Solids are drivento the bottom part of the furnace, from where they are collected by ascraper and conducted to a taphole provided at the bottom part of thefurnace. In the furnace according to said publication, various differentroasting steps cannot be realized in succession in continuous operation.

The object of the present invention is to eliminate drawbacks connectedto the roasting of anode slime and to realize a novel method and furnacestructure for treating anode slime in continuous operation. The furnaceaccording to the invention has a high capacity, and the material treatedaccording to the invention is suitable to be used as feed in Dorésmelting.

The method and roasting furnace according to the present invention arebased on continuous processing of anode slime and are particularly wellsuited to be combined with a process where anode slime is treated byhydrometallurgic methods after roasting.

In the method according to the invention, the slime containing valuablemetals and selenium is dried, roasted, sulfatized and cooled. Thetreatment includes steps that are carried out in succession, incontinuous operation, so that the slime is collected on the conveyor asa slime layer, which is then transported to be treated in successivedrying, roasting, sulfatizing and sulfuric acid removal and coolingunits.

The furnace according to the invention for continuously processing slimecontaining valuable metals and selenium comprises

a drying block, a roasting block, a sulfatizing and sulfuric acidremoval block as well as a cooling block, arranged in succession,

at least one conveyor or two or several conveyors arranged insuccession, which conveyors are arranged to convey the slime layerthrough the blocks from the beginning of the drying block to the end ofthe cooling block, and

a slime feeder device that is arranged to feed anode slime at a suitablespeed to the moving conveyor band.

The furnace according to the invention for treating anode slime enablesa controlled roasting of anode slime for removing selenium, and thesulfatizing of the slime for example for sulfatizing silver in the sameunit. In the arrangement according to the invention, the thickness ofthe anode slime layer can be maintained low, without still reducing thecapacity of the device. Material transfer to the slime layer and awaytherefrom is made effective, because the slime layer is thin, and thetemperature can be controlled, and consequently sintering that slowsdown material transfer can be prevented.

FIG. 1 is a side-view illustration of a continuously operated furnaceaccording to an embodiment of the invention.

FIG. 2 shows the cross-section of the furnace illustrated in FIG. 1,viewed from the top.

FIG. 3 shows the cross-section of the furnace illustrated in FIG. 1,viewed from the side.

The furnace comprises a band conveyor 11, onto which slime is fed from astorage silo by means of a feeder device 112 and conveyed to the firstblock 10 of the furnace. Onto the conveyor, there is charged apreferably uniform layer of slime with a thickness of less than 35 mm.The slime layer thickness is advantageously 10-20 mm. The speed of theconveyor 11 is selected for instance on the basis of the dimensions ofthe first furnace block that is the drying block 10, so that the layerreaches the desired moisture content when being discharged from thedrying block. The conveyor speed is advantageously within the range 2-20cm/min, preferably within the range 12-18 cm/min.

In the drying block 10, there is arranged a heat element 201 above theband conveyor 11 and consequently above the slime layer, through whichheat element the block gases are circulated. The block 10 is providedwith a fan 101 for blowing gases from the top and through the heatelement towards the slime layer surface. The drying block 10 is providedwith an exhaust pipe 111 for conducting the humid gas to the outlet ofthe block. Replacement air flows to the block 10 through air inlet pipes215. The state of the drying block is maintained at a slightunderpressure in order to prevent the gases from leaking out of thefurnace. The heating power of the heat element 201 is made such that theslime layer temperature in block 10 rises up to about 100° C., where theslime is dried.

The slime layer is conveyed on the band conveyor 11 from the dryingblock 10 to the roasting block 20 provided with at least one gas conduit320 for conducting the roasting gases into contact with the slime layer.Through the gas conduits 320, air or oxygen-enriched air is fed to abovethe slime for roasting the selenium by selenium dioxide gas. When theproperties of the slime require, the roasting output can at this stagebe improved by adding sulfur dioxide or sulfuric acid to the roastinggas. In the roasting block 20 the temperature of the slime layer israised, by means of a heating element 202 arranged in the roasting block20 up to 350-550° C. As for its mechanical structure, apart from the gasconduits, the block 20 of the roasting step may be similar to the dryingblock 10. The roasting block 20 is provided with a gas exhaust pipe 112,air inlet pipes 216, and a fan 102. The oxidation of selenium is anexothermic reaction and brings thermal energy to the roasting step,which means that the need for additional energy is low.

In the roasting block 20, selenium is oxidized according to the reactionformula (I) into selenium dioxide.Se+O₂(g)→SeO₂(g)  (I)Selenium oxide is recovered from block 20 by means of gas recoveryequipment arranged in the top part of the block (not illustrated). Inthe recovery equipment, there is created underpressure, so that gas iscollected in the recovery equipment, and in the roasting block 20 thereis maintained an underpressure that prevents the gases created in theblock from being emitted uncontrollably to the surroundings. From therecovery equipment, the gases are sucked into a recirculation tank bymeans of a solution-operated ejector. In the recirculation tank, theselenium oxide gas reacts with water resulting in selenium acid H₂SeO₃.

The slime is conveyed on a band conveyor 11 from the roasting block 20to the sulfatizing block 30, where the silver contained in the slime issulfatized. The employed sulfatizing reagent can be for example sulfuricacid in aqueous solution. The aqueous solution of sulfuric acid issprayed on top of the slime layer through nozzles 330 arranged in theblock. In this step, a possible selenium residue is removed as oxide,when the various selenides are broken owing to the effect of sulfuricacid. Also the copper possibly left in the slime is sulfatized. Likewisethe possible nickel oxide of the slime is sulfatized into nickelsulfate. Also chloride, slight quantities of which are always left inthe slime, is removed in this roasting step.

The reactions taking place in the sulfatizing block 30 follow thereaction formulas (II)-(IV):2 Ag+2H₂SO₄␣Ag₂SO₄+SO₂+2H₂O  (II)Cu+2H₂SO₄→CuSO₄+SO₂+2H₂O  (III)NiO+H₂SO₄→NiSO₄+H₂O  (IV)

The sulfuric acid sprayed on the slime cools the slime off. In thesulfuric acid treatment, the slime temperature is maintained, by meansof a heat element 203 arranged in the block 30, at 300-410° C.,preferably at 350° C., in which temperature excessive sulfuric acid isremoved, and the sulfatizing of silver is carried out most completely.In the sulfatizing step, some oxygen bearing gas, advantageously air, isfed into the block 30, so that the sulfatizing output is ensured.According to an embodiment of the invention, the temperature in theblock 30 is maintained within the range 250-300° C., which means thatthere is provided a sufficient underpressure for removing excessivesulfuric acid.

From the sulfatizing block 30, the slime is conveyed by the bandconveyor 11 to the last block 40 of the furnace, where the excessivesulfuric acid of the slime is removed, and the slime is cooled off. Thetemperature of the cooling block 40 is lowered, by means of coolingelements arranged therein, or by means of through circulation of coolingair, down to below 300° C. The sulfatizing block 30 is provided with agas exhaust pipe 113, air inlet pipes 217 and a fan 103. Respectively,the cooling block 40 is provided with a gas exhaust pipe 114, air inletpipes 218 and a fan 104. The slime is conveyed out of the furnace andconducted to a silo for intermediate storage.

In a furnace according to the invention, the band conveyors areadvantageously made of steel. There can be only one band conveyor, orelse several band conveyors in succession, arranged so that the slimelayer is transferred from one band to another without delay. The speedsof the band conveyors can be separately adjusted, in which case thedelay time of the slime layer and the layer thicknesses in differentblocks can be adjusted block by block, to be best suitable in eachsub-process. Also the lengths of the various blocks in the proceedingdirection of the conveyor can be different, in which case the processingtime in each block can be arranged to better fulfill the demands.

According to an embodiment of the invention, it is not necessary tocarry out the sulfatizing step. In that case the furnace can be builtshorter, and the sulfatizing block can be left out.

According to another embodiment of the invention, the sulfatizing stepis realized in a kiln furnace arranged in connection with the blocks, sothat the material transfer between the solids and the gases becomes moreeffective. The slime layer is fed from the roasting block to thesulfatizing kiln furnace by the conveyor band, and the sulfatized slimelayer is transferred to another conveyor that conveys the sulfatizedsolids to sulfuric acid removal and further to the cooling block.

EXAMPLE

Anode slime, with a humidity of 25%, was treated in a continuouslyoperated roasting furnace according to the invention. The total lengthof the furnace was 16 meters. The width of the steel conveyor band was 2meters. The furnace capacity was about 225 kg/h dry slime. The length ofeach of the four blocks was about 3 meters. The layer of anode slime fedonto the steel band conveyor was 10-20 mm thick.

In the drying step, there were used heat elements with a power of 80 kW,so that the rate of evaporated water was about 75 kg per hour. In theroasting step, the slime temperature was raised up to 450° C. at aheating power of 80 kW. In the third step the slime was cooled down toand maintained at 350° C., in which case the power demand was about 50kW. In the last step, the slime was cooled down to 300° C., and therequired power was about 50 kW. The power of the four furnace fans was 2kW each.

The required quantities of chemicals per one kilo of dry slime were:0.75 kilo sulfuric acid and 45 grams oxygen.

For a man skilled in the art, it is obvious that the various embodimentsof the invention are not restricted to the appended examples anddrawings only, but may vary within the scope of the appended claims.

1-10. (canceled)
 11. A method for treating slime containing valuablemetals and selenium, in which method the slime is dried, roasted,sulfatized and cooled, wherein the method includes steps to be carriedout in succession and in continuous operation, so that the slime isformed as a slime layer on the conveyor and is conveyed to be treated insuccession to successive drying, roasting, sulfatizing and sulfuric acidremoval and cooling units.
 12. A method according to claim 11, whereinthe thickness of the slime layer is under 35 mm, preferably 10-20 mm.13. A method according to claim 11, wherein in the drying unit, theslime layer is heated up to 100° C., and the removal of moisture fromthe slime layer is boosted by a fan.
 14. A method according to claim 11,wherein in the roasting unit, the temperature of the slime layer israised up to the range 350-550° C., and that the material transfer inthe slime layer is boosted by a fan.
 15. A method according to claim 11,wherein in the sulfatizing and sulfuric acid removal unit, thetemperature of the slime layer is set within the range 250-350° C., andthe employed sulfatizing reagent is sulfuric acid.
 16. A furnace fortreating slime containing valuable metals and selenium in continuousoperation, in which furnace the slime is roasted for removing theselenium and for sulfatizing the metals, wherein the furnace comprises adrying block, a roasting block, a sulfatizing and sulfuric acid removalblock and a cooling block, arranged in succession, at least one conveyorthat is arranged to convey the slime layer through the blocks from thebeginning of the drying block to the end of the cooling block, and aslime feeder device, that is arranged to feed anode slime at a suitablespeed to the moving conveyor band.
 17. A furnace according to claim 16,wherein there are two or more conveyors arranged in succession.
 18. Afurnace according to claim 16, wherein the feeder device can be arrangedto form an anode slime layer with a thickness of 35 mm or thinner ontothe slime conveyor.
 19. A furnace according to claim 16, wherein in thedrying block, in the roasting block and in the sulfatizing and sulfuricacid removal block, above the conveyor, there is in each block arrangeda heater and a fan above the heater.
 20. A furnace according to claim16, wherein the cooling block is provided with at least one coolingelement for cooling the slime layer.